During transport thereof, in particular, the objects transported are subject to physical and mechanical stresses such as acceleration, shocks, vibrations, heating, etc. These stresses can have serious consequences when the objects are costly and complex items of equipment which, after transport thereof, contribute to missions the failure of which may cause loss of human lives or of very expensive assemblies, which is the case with fields involving hostile or very stressful media: aeronautics, astronautics, underwater navigation, for example.
It is very important to know these stresses as well as the time when they occur.
Knowing the stresses makes it possible to discard the equipment concerned or to subject it to thorough tests such as those performed at the time of acceptance.
Moreover, knowing the time when the phenomenon arises makes it possible to avoid a repetition of it, by guarding against it for example by better packaging, and also to determine liabilities.
But transport and handling are not the only circumstances during which serious stresses may arise and which it is good to monitor.
It may be useful to know that a motor has been running with temporary overheating, although it has not given rise to any breakdown or even any alert.
Seals may have undergone stresses likely to reduce their future strength, without those responsible having the least actual and objective information allowing them to decide a priori to replace them.
For this, an important characteristic expected of an apparatus for monitoring stresses undergone by equipment is its autonomy.
Autonomous apparatuses for monitoring objects during transport thereof are already known.
J. KORELUS in the document QZ 25 (1980), vol. 4, "The transport shock measuring instrument--an accessory protective device for valuable goods," describes an apparatus for recording shocks undergone by valuable objects during transport thereof. This apparatus comprises three accelerometers the signals from which are added, an analogue-to-digital converter, a control logic device and storage registers, as well as a printer which displays, opposite the level of measured acceleration, the date and time of recording. The recording threshold is adjustable.
A major drawback of this apparatus is the use of a printer, a device which is not versatile, consumes a very high amount of energy and is relatively unreliable. Moreover, it is limited to a single method of measurement.
The patent EP-A-0 235 534 describes a recorder comprising:
three accelerometers, PA0 clock means, PA0 memories for storing programme instructions and data representing numerical signals generated by the three accelerometers and by the clock means, PA0 transmission means for communicating numerical data to an external device, PA0 microprocessors connected to the above means, PA0 a power supply. PA0 at least one sensor placed on each door of each container; PA0 an autonomous active module placed inside the vehicle connected to the sensors, which comprises power supply means, means for processing the data which are transmitted to it such as the location of the vehicle, opening of the doors or on other parameters relating to the journey of the vehicle etc. and means for generating a reference signal at regular intervals of time; PA0 a central data processing unit independent of the vehicle for managing and checking the data transmitted or received by the module; PA0 means of connection between the vehicle and the central processing unit such as a radio or a telephone. PA0 the rules of the job are stored in a volatile memory and these rules are saved, while the sensor or sensors are in a so-called "suspended" state, PA0 the date and time at which the job is started are stored, that is, the moment when the sensor or sensors are changed from the suspended state to a so-called "monitoring" state, PA0 a recorder assembly is changed from the monitoring state to a so-called "acquisition" state corresponding to effective data recording, when the signal from a sensor exceeds a certain threshold, PA0 signals representing stresses and/or conditions of use of the equipment are sensed, and at least some data are recorded in real time according to a set of rules selected by the user and constituting a specific job, PA0 access to the rules of this job is prohibited for third persons and, in particular, for an operator having custody of the equipment, and access to the data recorded according to the rules of the job is given at the exclusive request of the user. PA0 several sensors are placed and the data of only some of them are recorded; PA0 any event which may have the effect of neutralizing or activating each of the sensors placed is recorded; PA0 on the one hand upon request and on the other hand before any recording operations, individual functioning of all the sensors necessary for the specific job is checked; PA0 the duration of stresses and/or conditions of use is calculated for several measurement values determined by the user. PA0 sensors connected to the recorder assembly have a circuit for checking their functioning, for bringing about recording in the memory of data indicating that they are working or not working; PA0 it comprises circuits for backing up the memory for storing the rules, the memory for storing sensed data, as well as the time stamp when the device is in a so-called "suspended" state or when the power supply (1041) of the device is insufficient; PA0 it is in a so-called "monitoring" state when its memories effectively contain program and job data, the device comprising circuits which make it possible, in the monitoring state, to deselect members connected to the microprocessor such as memories, time stamp, keyboard, display screen, sound warning device or others, for the purpose of saving the power supply; PA0 it is in a so-called "acquisition" state when any of the sensors receives a signal with a value greater than that of a predetermined threshold stored in the memory; PA0 it comprises three accelerometers mounted in a trihedron to provide signals to be filtered by three sets of three different bandpass filters, mounted in parallel, the signals from the identical filters of each set to be added by means of circuits before being transmitted to three inputs of the microprocessor; PA0 it comprises at least six sensors including three accelerometers and at least three other sensors such as a temperature sensor, a pressure sensor and a humidity sensor, all connected to the same number of inputs of the microprocessor; PA0 it comprises at least one pilot lamp which is lit up if the value of a recorded signal is greater than a predetermined programmed threshold and, if occasion arises, when a person acts on a means for control of lighting up.
This recorder reads cyclically the signals picked up, and stores them in a memory when their amplitude exceeds a predetermined threshold.
This apparatus therefore necessarily works permanently, which greatly limits its autonomy because it consumes so much energy that it is unrealistic to envisage using it for long periods, whether transport, handling or operation is concerned.
Moreover, no processing, particularly compression, being carried out on the signal provided by the accelerometers before storing it, a signal with a duration of 50 milliseconds occupies 960 octets in the memory, and the storage capacity of the apparatus is limited to about a hundred signals.
Patent document DE-A-3 643 203 describes a device including an accelerometer (or acceleration sensor) and a recorder with memory of which the electronics comprise an amplifier, a digitizer and a microprocessor. These electronics are activated, that is, supplied with power, only when the sensor delivers a signal indicating an acceleration the value of which exceeds a programmable threshold. This apparatus activated by the phenomenon to be measured needs a certain time to adopt its operating conditions, and there is a risk of missing the start of rapid phenomena. Moreover, the recorder described does not process signals before storing them. The data are not protected and the apparatus is not "discreet", for it emits a warning sound signal systematically every time the threshold is exceeded.
We might also cite the document U.S. Pat. No. 4,750,197 which describes a built-in device for protection against theft of objects shipped in containers placed inside means of transport such as vehicles.
This device includes:
This device which is primitive and complicated does not make it possible to ensure monitoring of the objects transported in respect of the stresses to which they are subject (mechanical strains, variations in physical parameters such as temperature, humidity, etc.).
Furthermore, this device is operational only after a check by the central processing unit, once and for all during the whole voyage, that all the sensors and the module are working properly, that is, before departure of the vehicle, which makes the reality of monitoring uncertain and random, since possible breakdowns of the sensors are not detected.
The central processing unit periodically checks the state of operation of the module on board, which does not make it possible to detect the unpredictable and sudden trend of variation of a parameter.
This principle supposes that the events monitored are necessarily likely to last longer than an interval between two checks, which is the case in particular with burglaries. The result is that monitoring is not ensured permanently, which may be highly prejudicial to the objects transported, particularly if they are fragile.